Healthy coral reefs provide their adjacent coasts with substantially
more protection from destructive tsunami waves than do unhealthy or
dead reefs, a Princeton University study suggests.
Initially
spurred by the tsunami that devastated the coastlines of the Indian
Ocean two years ago, a team of scientists developed the first-ever
computer model of a tsunami strike against a reef-bounded shoreline,
using a volcanic island as an example. The model demonstrates that
healthy reefs offer the coast at least twice as much protection as dead
reefs. The finding provides the first quantitative confirmation of a
widely held theory regarding the value of living coral reefs as a
defense against tsunami waves, which are often generated by powerful
undersea earthquakes.
Princeton professor Michael Oppenheimer
said his team's work will give scientists the ability to quantify how
much any given reef will benefit its particular stretch of coast.
"Healthy
reefs have rougher surfaces, which provide friction that slows the
waves substantially in comparison with smoother, unhealthy ones," said
Oppenheimer, the Albert G. Milbank Professor of Geosciences and International Affairs.
"Scientists had never before studied this effect by the numbers, nor
had they ever analyzed it over a wide variety of coastal shapes. This
study provides yet another motivating factor for protecting the
planet's coral reefs from degradation."
The team's findings
appear in the Dec. 14 edition of the journal, Geophysical Review
Letters. In addition to Oppenheimer, other team members include Robert
Hallberg, who is head of the Oceans and Climate Group at the National
Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Lab,
and Catherine Kunkel, who is the paper's lead author. Kunkel
spearheaded the work during her senior undergraduate year at Princeton,
from which she graduated with a physics degree in June.
Though
anecdotal observations of reefs' effects on tsunami abounded after the
2004 Indian Ocean strike, Kunkel said, it was difficult to form any
real conclusions because so many of these observations came after the
fact. The goal of this study, which began as Kunkel's senior thesis
project, was to provide a systematic framework by which to examine the
assumption that healthy reefs protect shorelines more effectively.
"For
our purposes, we assumed that the health of the reef would only be
important in terms of the drag it exerted on the wave," said Kunkel,
who is currently working as a research assistant at Tsinghua University
in China. "If you have a healthy reef, it has lots of live coral
branching out, sticking a lot of small obstacles into the water. A dead
reef, on the other hand, is not as rough -- it tends to erode and
exerts less drag on the wave."
A turbulent mountain of water
crashing over a complicated rough surface presented Kunkel with a
number of obstacles for her own study -- specifically, how to find a
way to express each of these effects with a mathematical formula that a
computer could employ to simulate it. Different complex parameters had
to be considered one by one: the width and depth of the reef; the
roughness of its surface; the size of the lagoon behind it; and the
slope of the coast beyond. And the overarching element was the wave
itself and its interaction with all these obstacles. Eventually, Kunkel
found a set of equations that provided a limited but comprehensive
picture of a tsunami strike.
"We had to idealize a number of
factors, because we wanted to create a model that could be used for a
general shoreline," Kunkel said. "For example, we had to consider a
perfectly even ocean floor, because uneven ones can funnel a wave into
a certain area."
Despite the limitations of the model, Oppenheimer said it provides a sound basis for the team's conclusions.
"The
general conclusion is that a healthy reef might provide twice as much
protection as a dead one," he said. "This could translate into sparing
large sections of inshore area from destruction."
Because coral
reefs are dying from rising ocean temperatures, increasing ocean
acidity, and direct human damage, Oppenheimer said the findings offer
yet another reason to protect these fragile offshore ecosystems.
"This
study shows yet another way that protecting the environment relates to
humanity in a very tangible way," he said. "Villages get built behind
coral reefs for good reasons, and this is one of them."
Kunkel
said that she hoped the study would inspire other scientists to
continue the research by obtaining more observational data.
Incorporating such data into the team's theoretical model, she said,
would then allow scientists to plan better for future tsunami strikes
along local coastlines.
"We now have a basic idea of what
variables are important, but if you want to quantify the effectiveness
of a barrier reef around a particular island, you'd want to model that
island directly," Kunkel said.
Abstract
Coral reefs reduce tsunami impact in model simulations
Catherine M. Kunkel, Robert W. Hallberg, and Michael Oppenheimer
Significant
buffering of the impact of tsunamis by coral reefs is suggested by
limited observations and some anecdotal reports, particularly following
the 2004 Indian Ocean tsunami. Here we simulate tsunami run-up on
idealized topographies in one and two dimensions using a nonlinear
shallow water model and show that a sufficiently wide barrier reef
within a meter or two of the surface reduces run-up on land on the
order of 50 percent. We studied topographies representative of volcanic
islands (islands with no continental shelf) but our conclusions may
pertain to other topographies. Effectiveness depends on the amplitude
and wavelength of the incident tsunami, as well as the geometry and
health of the reef and the offshore distance of the reef. Reducing the
threat to reefs from anthropogenic nutrients, sedimentation, fishing
practices, channel building and global warming would help to protect
some islands against tsunamis. Kunkel, Hallberg and M. Oppenheimer
(2006), Coral reefs reduce tsunami impact in model simulations,
Geophys. Res. Lett., 33, L23612, doi:10.1029/2006GL027892.
